1. Field of the Invention
The present invention relates to an automatic music transcription apparatus and program.
2. Discussion of Background
Since the act of writing down audio information taken from a music CD or the like, which is called music transcription, can be done only by people having musical knowledge and special capabilities such as perfect pitch, it has long been studied to have a computer or the like do the work.
One factor that makes it difficult to transcribe music automatically by a computer is overtones of a note produced by a musical instrument.
When a single note is produced by a musical instrument, the frequencies of the fundamental note (fundamental wave) and a plurality of overtones (harmonics) corresponding to the degree of highness (pitch) of the sound are generated at the same time. Although the overtone frequencies are usually integer multiples of the fundamental note, it is known that the frequencies of high-order overtones of the piano are not integer multiples of the fundamental note.
The ratio of the power of each overtone to the power of the fundamental note depends on the musical instrument. Even in the same musical instrument, the power ratio varies with the pitch of the sound and with time after the key is depressed or the sound is produced. Strictly speaking, each produced sound has a different power ratio, depending on the way the key is touched or the way the sound is produced (tonguing and the like), even if the same note is made by the same instrument.
The state of a single note is complicated, as described above, and when a plurality of notes are sounded simultaneously, the state becomes even more complicated. If some fundamental notes or overtones of the plurality of the simultaneously produced notes have close frequencies, the powers of the fundamental notes or overtones change because the phases cancel out each other or overlap with each other.
In automatic music transcription, the pitch of an instrumental note is extracted by detecting the frequency of the fundamental note of the instrument. However, because the overtone-to-fundamental power ratio varies with many conditions, it is not easy to judge whether the note is a fundamental note or an overtone. This fact has made it difficult to transcribe music automatically.
One method to eliminate those overtones is disclosed in JP-A-2000-293188, for instance. On the assumption that the power ratio generally depends on the musical instrument, the method disclosed in this reference determines whether a frequency (comparison frequency) higher than a frequency of interest is an overtone of the frequency of interest, and if yes, reduces the sound volume of the comparison frequency by a certain ratio and adds the reduced sound volume to the sound volume of the frequency of interest under certain circumstances.
If the power ratio almost depends on the musical instrument, the method described above would be effective. Actually, many musical instruments have power ratios greatly varying depending on ranges, so that overtones might not be properly eliminated by a certain ratio in some ranges.
The conventional structure reduces the sound volume of the comparison frequency (overtone) by a certain ratio, but the comparison frequency may contain the sound volume of overtones of another note sounding at the same time. The sound volume of the comparison frequency should not be reduced by a certain ratio; instead, the sound volume of the frequency of interest (fundamental note) multiplied by a ratio depending on the order of the overtone of the comparison frequency should be reduced from the sound volume of the comparison frequency.